Techniques are known for controlling the operating temperature of devices that generate heat during operation and require cooling to keep their temperature below a threshold limit. Examples of systems employed for attaining this end are shown in the following prior art.
U.S. Pat. No. 4,126,269 (Bruges) discloses a series of fans spaced along a flow path in a ventilated housing which cools electronic devices therein with a flow of filtered air. Each fan has a temperature sensor for emitting a signal when a temperature threshold limit is reached, so indicating inadequate cooling due to a clogged filter or the stopping or reduction in speed of the fan.
U.S. Pat. No. 4,648,551 (Thompson et al.) discloses a blower motor system for an indirect heat exchange flow, fuel gas/air furnace, that adjusts the fuel gas feed and the pressure delivery fan for intake air to be preheated by heat exchange with an already combusted fuel gas/air mixture. After condensing water from the heat exchanged combusted mixture, the latter is removed under suction fan operation. Flow adjustments are made, e.g., to compensate for flow variation due to dirty filters, closed ducts, reduced line voltage and increased motor temperature, so as to maintain appropriate air delivery for a specified air temperature rise in the system. The adjustments are controlled by pressure sensors for the combusted mixture and intake air, respectively, in the heat exchanger, and a temperature sensor for the preheated air to effect its precooling in a separate unit if excessively preheated in the heat exchanger prior to combustion.
U.S. Pat. No. 4,659,290 (Kundert) discloses a fan for cooling electronic components with a main air flow, which has a speed controller whose circuit is in a chamber on the fan hub through which a minor air flow, representative of the main air flow, passes for sensing by a temperature sensor. The fan is adjusted as a function of the air temperature relative to a threshold limit, to increase its speed above a minimum when the air temperature exceeds the limit, and to return it to minimum speed when the temperature returns below the limit. An alarm system signals overheating, e.g., due to a clogged filter, even if the fan functions normally.
U.S. Pat. No. 4,722,669 (Kundert) discloses a speed controller for a fan that cools electronic devices, in which an exhaust air temperature sensor controls the fan speed to keep the devices at a constant temperature over a range of varying inlet air temperatures. Thus, fan speed is adjusted relative to changes in inlet air temperature as sensed by the exhaust air temperature.
In certain thermal printers, an array of thermal print head elements is disposed to drive dyes from a dye-bearing web into a dye receiving sheet. In others, resistive elements of the print head are selectively energized to create images directly on thermally sensitive media. In these units, only a small portion of the energy fed to the elements generates the images, the major energy portion remaining in the thermal head which functions as a heat sink. This excess energy stored in the head raises its temperature, so that the effective image density increases with increasing head temperature. Often, the unit has cooling means such as a fan to remove excess heat from the head. It is known that print quality improves if the head is kept at a temperature higher than ambient.
Typically, during the line printing time for printing each line of images, the thermal print head elements are selectively energized by constant current pulses of predetermined time duration or pulse width. A control circuit generates enable signals of selective time duration which are used to vary or modulate the pulse width of the energizing current. In this way, the heating elements are energized with an enable width corresponding to the selective time duration of the enable signals during the line printing time, for controlled printing of the images. The heat generated in the head increases with increase in the proportion that the enable time (power energizing time) bears to the line printing time.
A variety of means has been used to sense the power delivered to the thermal head. Other sensing means have been disclosed to monitor the head temperature. These sensors are typically connected to electrical control systems that modulate the enable width of the element energizing current, the head voltage, or a cooling fan or other cooling means, in an effort to assure that the resulting print density remains constant.
U.S. Pat. No. 4,797,837 (Brooks) discloses controlling a thermoelectric heat pump to cool intermittently the thermal head of a thermal printing device having a plurality of heating elements that generate images on a responsive imaging material. The head temperature is sensed and compared with a reference temperature to determine if heat pump operation should be initiated or halted.
Commonly assigned U.S. Pat. Nos. 4,745,413 (Brownstein et al.) and 4,912,486 (Yumino), disclose typical thermal printing devices having a thermal printing head with a plurality of heating elements used to generate images on a responsive imaging material.
In all these systems, uneven heating and abrupt changes in delivered power can cause the associated temperature sensors to operate sub-optimally.
These known arrangements use complicated and costly means to attain temperature control or lack adequate means for such control.
It is desirable to have a thermal printing device with is-enthalpic control and a method of operating such a device under is-enthalpic control, to cool its printing head so that the net energy in the head remains substantially constant, independently of cooling effect changes caused by changes in ambient cooling fluid, such as air temperature, humidity and barometric pressure changes, or filter clogging.